1. Field of the Invention
The present invention relates generally to gain equalization suitable for optical fiber communication using wavelength division multiplexed signal light including a plurality of channels of optical carriers having different wavelengths, and more particularly to combined gain equalizers and an optical transmission system having the combined gain equalizers.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying signal light has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pump light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength of 1.55 .mu.m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by a gain characteristic (wavelength dependence of gain) which is represented by a gain deviation or gain tilt. For example, in an EDFA, a gain deviation is produced at wavelengths in the vicinity of 1.55 .mu.m. When a plurality of EDFAs are cascaded to cause accumulation of gain deviations, an optical SNR (signal-to-noise ratio) in a channel included in a band giving a small gain is degraded.
To cope with the gain deviation of an optical amplifier, a gain equalizer configured by a passive optical component such as an optical filter is used. Before a degradation in optical SNR in a certain channel becomes excessive due to accumulation of gain deviations, gain equalization is performed by the gain equalizer provided at a suitable position.
In the case that one kind of gain equalizer is used, the accuracy of gain equalization is low on the basis of the fact that an optical filter as the gain equalizer generally has a simple periodic loss characteristic (wavelength dependence of loss) or a loss characteristic having a single peak. While it has been proposed to combine two kinds of gain equalizers, the accuracy of gain equalization is not necessarily sufficient in some case. Further, it may be proposed to combine five or more kinds of optical filters and design a loss characteristic matching the gain characteristic of an optical amplifier. In this case, however, the gain equalizer is complicated in configuration, and the loss becomes high. The use of one kind of gain equalizer or the use of two kinds of gain equalizers in combination will be hereinafter described in the section of DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS in comparison with the present invention.